Adjuvant and other effects of BCG vaccine and its influence on the epidemiology of new coronavirus disease COVID-19

The live BCG vaccine, causing a complex response of both innate and cellular as well as humoral adaptive immunity, is a biological adjuvant. It serves as a trigger for a "trained" immune system response, characterized by the activation of monocytes, macrophages, natural killer cells, and lymphoid elements of inborn populations, all contribute to the early activation of non-antigen-specific protective programmes of the body fight against a number of viral, fungal, protozoan infections and neoplastic clones. One of the infections, altered by BCG vaccination, may be COVID-19. The pathogenesis of the development of acute interstitial pneumonia/respiratory distress syndrome caused by COVID-19 is characterized by the triggering of excessive systemic action of inflammatory mediators, in particular, cytokines, due to violation of the focal inflammatory barriers. Gamma-interferon, produced by lymphocytes after BCG vaccination, modulates the activity of a number of interleukins, which in turn may attenuate course of COVID-19 by reducing the activity of IL-12 and IL-18 -dependent reactions. There is an antigenic cross-reaction between the peptides from causative agents of mycobacterioses and SARS-CoV2 because of their proteins’ homology. Unlike many adjuvants, BCG decreases the incidence of lymphoid malignancies and its effect on various autoimmunopathies is different, not necessarily harmful. The peculiar character of BCG vaccination effect may be related to its very early impact on immature immune system and symbiotic character of host-BCG interactions. Geo-epidemiological data on the relationship between the historical practice of using BCG vaccination in different countries and the current incidence of new coronavirus infection and mortality from it are presented. Historically, the medical and social reasons for the different national policies of health authorities regarding the use of the BCG vaccine are considered.